RFR: 8076290: JCK test api/xsl/conf/string/string17 starts failing after JDK-8074297

[Aleksej Efimov]

Hi,

Please review a fix for JCK test failure [2] that was caused by slightly 
incorrect fix for JDK-8074297 [3].
The problem is that the length of required string is incorrectly 
calculated when the start index is less or equal than zero: The pushed 
fix for JDK-8074297 assumes that length are specified from the first 
string character, but it should be counted from the specified start 
index - two added test cases for substring test illustrates it - one for 
string with supplementary characters, another one without.
All JCK xml related tests passes (including the failed one).
Tests set: api/xsl api/javax_xml api/org_xml xml_schema api/xinclude.
Test results: passed: 35,342

Thank you,
Aleksej

[1] Webrev: http://cr.openjdk.java.net/~aefimov/8076290/00/
[2] JBS: https://bugs.openjdk.java.net/browse/JDK-8076290
[3] JDK-8074297: https://bugs.openjdk.java.net/browse/JDK-8074297